As dimensions have shrunk, lithographic engineers have resorted to various methods to reduce the size of apertures passing through interlevel dielectrics such as growing a polymer on the vertical surface of a resist hole (Relacs); a reflow of resist; a negative etch bias in transferring the contact hole to the substrate; and deposition of a sidewall spacer on the inside of the contact hole.
The negative etch bias often introduced a slope in the profile of the aperture, resulting in poor control of the aperture size.
The spacer approach introduced an additional etch step.
Various approaches have been shown in patents for depositing layers of oxide from the liquid phase, such as U.S. Pat. No. 6,251,753, U.S. Pat. No. 6,653,245, and U.S. Pat. No. 5,776,829 incorporated by reference.